Replace NULL with nullptr in all C++ files.

webrtc/tools/converter/rgba_to_i420_converter.cc

Index: webrtc/tools/converter/rgba_to_i420_converter.cc
diff --git a/webrtc/tools/converter/rgba_to_i420_converter.cc b/webrtc/tools/converter/rgba_to_i420_converter.cc
index 6b1056fae4cb3b4dc6bc45c0172e16db11ba8aa8..59c15ad2e5d7b092baded6b18d67854dd5771e53 100644
--- a/webrtc/tools/converter/rgba_to_i420_converter.cc
+++ b/webrtc/tools/converter/rgba_to_i420_converter.cc
@@ -65,8 +65,8 @@ int main(int argc, char** argv) {
   }
   parser.PrintEnteredFlags();
 
-  int width = strtol((parser.GetFlag("width")).c_str(), NULL, 10);
-  int height = strtol((parser.GetFlag("height")).c_str(), NULL, 10);
+  int width = strtol((parser.GetFlag("width")).c_str(), nullptr, 10);
+  int height = strtol((parser.GetFlag("height")).c_str(), nullptr, 10);
 
   if (width <= 0 || height <= 0) {
     fprintf(stderr, "Error: width or height cannot be <= 0!\n");